1. Field of the Invention
The present disclosure relates to an image forming apparatus including a rotating polygonal mirror having a plurality of reflection surfaces, and more particularly to an image forming apparatus that identifies the reflection surface on which a light beam is incident.
2. Description of the Related Art
An image forming apparatus has been known which deflects a light beam emitted from a light source by using a rotating polygonal mirror (hereinafter, polygonal mirror) including a plurality of reflection surfaces so that the deflected light beam scans a photosensitive member to form an electrostatic latent image on the photosensitive member. Characteristics of the polygonal mirror such as reflectance of each reflection surface and an angle (plane tilt) thereof with respect to the rotation axis vary depending on cutting accuracy during manufacturing. Variations in the manufacturing accuracy therefore need to be corrected by identifying the reflection surface on which the light beam is incident and making corrections according to the identified reflection surface.
As a method for identifying the reflection surface on which the light beam is incident, Japanese Patent Application Laid-Open No. 2006-142716 discloses an image forming apparatus that identifies the reflection surface on which the light beam is incident by using variations in the generation period of a beam detecting signal. The BD signal is generated by a BD (Beam Detector) that receives the light beam deflected by each of the plurality of reflection surfaces during one rotation. According to the method using variations in the period of the BD signal, disclosed in Japanese Patent Application Laid-Open No. 2006-142716, the light beam needs to be emitted in consideration of the timing at which the light beam deflected by the polygonal mirror is incident on the BD. In such an identification method, it is necessary that the polygonal mirror is rotating at approximately constant speed.
In the method disclosed in Japanese Patent Application Laid-Open No. 2006-142716, the BD signal needs to be generated and the reflection surfaces are identified after the polygonal mirror has reached approximately constant rotation speed. According to such a method, the time to start image formation (first copy Output time (FCOT)) increases because the step of generating the BD signal is required.